From the info given, here's what I've deducted.
I Googled your rig and if I have the model # correct it has integrated video.
It also has an AGP slot for upgrades.
You are trying to use the PCI slot for a video upgrade.
With the above on the table, here's what I would do:
1. Disable the on-board GPU if the Dell BIOS allows for this.
2. Remove the old PCI video card drivers
3. See if I can return the new PCI video card for an AGP version that is compatible with AGP 4x
If you can dig into the Dell archives, see if you can find the options list and see what AGP cards they were offering for your rig.
You know going in that those cards have been tested in your rig.
That would have been my Step One
The only reason to turn to PCI for video is if you don't have an AGP slot or you have a problem with your AGP slot.
AGP was designed for video.
The 5200 PCI may be wanting more power than your PCI bus can deliver. You will have a better chance using the AGP slot.